1. Field of the Invention
The present invention relates generally to a method of depositing thin films. In particular, the invention concerns a method of growing elemental metal thin films by Atomic Layer Deposition (ALD) using a boron compound as a reducing agent.
2. Description of the Related Art
The integration level of components in integrated circuits is increasing, producing a need for smaller components and interconnects. Design rules are dictating a feature size less than or equal to 0.2 xcexcm. This makes complete film coverage on deep vias difficult to obtain.
Integrated circuits contain interconnects that are conventionally made of aluminum. Today, copper is replacing aluminum because it has lower electrical resistance and better electromigration resistance than aluminum.
Chemical Vapor Deposition (CVD) has been commonly used to produce metal films. In CVD, the source materials are typically fed into a reaction space together, where they react when brought into contact with a hot substrate. Thus, the growth rate of the metal film depends in part upon the concentration of the different source materials in the reaction space. Additionally, the temperature of the substrate affects the rate of deposition. In thermal CVD a single source chemical can be thermally decomposed near the substrate.
Atomic Layer Deposition (ALD) is an advanced alternative to CVD. The ALD method is based on sequential self-saturating surface reactions and has been described in detail in U.S. Pat. Nos. 4,058,430 and 5,711,811. Source chemicals are pulsed into the reaction chamber in an inert carrier gas. The pulses of source chemical can be separated from each other by a purging flow of inert gas. The separation of the source chemicals and the proper choice of source chemicals prevents gas-phase reactions between gaseous reactants and enables self-saturating surface reactions. This allows for film growth without strict temperature control of the substrate or precise dosage control of the reactants. Surplus reactants and byproducts are removed from the chamber, such as by a purging flow of inert gas, before the next reactive chemical pulse is introduced. Undesired gaseous molecules are effectively removed from the reaction chamber by keeping the gas flow speeds high. The purging gas pushes the extra molecules towards the vacuum pump that is used to maintain a suitable pressure in the reaction chamber. Thus, ALD provides for rapid, uniform, controlled film growth.
While ALD has been used to produce both elemental and compound thin films, there are a number of drawbacks to the methods that have been used. Thus, a need exists for improvements in the production of metal thin films.
In one embodiment, the present invention provides a method of growing elemental thin films on a substrate by an atomic layer deposition (ALD) type process. The method comprises introducing vapor phase pulses of at least one elemental source compound and at least one boron source compound into a reaction space that contains a substrate on which the thin film is to be deposited.
The vapor phase pulses are alternately introduced in a cycle. Each cycle comprises introducing an elemental source compound into a reaction space containing a substrate, removing any gaseous compounds from the reaction space, introducing a boron source compound into the reaction space, and removing any gaseous compounds from the reaction space. The elemental source compound preferably reacts with the surface of the substrate producing a surface bound elemental compound. Preferably the boron source compound is capable of reducing the surface bound elemental compound into elemental form.
In the preferred embodiment a metal source compound is used and an elemental metal thin film is grown on the substrate. The metal source compound and boron source compound are fed into the reaction chamber with the aid of an inert carrier gas. An inert gas may also be used to purge the reaction space after each pulse of metal source compound and boron compound.
In one embodiment the boron compound contains at least one carbon atom and the elemental source compound comprises at least one metal selected from the group consisting of Cu, Ag, Au, Pd, Rh and/or Ir. In another embodiment the boron compound contains no carbon atoms and the elemental source compound comprises at least one metal selected from the group consisting of Cu, Ag, Au, Pd, Rh, Ir, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W.
In accordance with one aspect of the invention, an electron conductor is produced by an ALD type process wherein a boron compound is used to reduce a surface bound elemental compound to its elemental state. In accordance with another aspect of the invention, an interconnect is produced in an integrated circuit by depositing a metal thin film by an ALD type process wherein a boron compound is used to reduce a surface bound metal compound to its elemental state. In yet another embodiment a metal seed layer is grown on a substrate by growing an elemental metal thin film on a substrate by an ALD type process wherein a boron compound is used to reduce a surface bound metal compound to its elemental state.